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Aviation History
1949
1949 - 0345.PDF
February 24th, 1949 FL1GH T 223 By W. A. JOHNSON, M.A. Multi-point "Dischargers " can be Designed to Suppress Radio Interference Fig. 3. In the absence of pointed dis- chargers, there can be a large build-up of charge on an aircraft Static Discharges from Aircraft THE effects of static discharges from aircraft, andlightning strikes to aircraft, have received increasingprominence with the revival of civil aviation sincethe war. The main phenomena observed include minor damage to aircraft structures, in the form of fused pits and holes in projecting parts such as wing tips; corona or glow discharges, and severe interference with the radio installa- tion. Damage to engine hearings has been reported, but there is no evidence to show that this is caused by anything other than mechanical defects, or the results of lightning. It is important to differentiate between lightning discharges to and static discharges from aircraft, The former is due to equalization of charges between clouds, or between cloud and ground, the aircraft forming part of the discharge path. It may be that in a region of a high potential gradient the aircraft may disturb the potential distribution sufficiently to initiate a discharge. The current flow from the aircraft structure may well be several hundred amperes, lasting for several milliseconds, and the amount of charge transferred may be large, up to 100 coulombs. While peak currents of 10,000 amperes are not unusual in lightning discharges, such large currents only last two or three microseconds. There are no known means of eliminating the hazard from light- ning, other than by keeping away from dangerous areas. The fitting of radar-type cloud warning sets will be of great use in avoiding the worst storm centres. Laboratory tests have. shown that a spark discharge amounting to 2 coulombs is necessary to produce a notice- able pit in metal structures. Fig. 1 shows the spinner of an aircraft which was struck by lightning while towing a glider. The stroke was observed to pass from the spinner through the aircraft, tow rope and glider, and there is no doubt as to its authenticity. Fig. 2 shows an enlarged view of the pit marks on the spinner. It must, therefore, be concluded that all such damage is compatible with currents of the magnitude of lightning flashes, and are not due to corona discharges or " precipitation static." Fortunately records show that the risk of fatal damage to aircraft by lightning is relatively remote, and the greater hazard is loss of radio communication due to corona effects, particularly in bad visibility conditions. The other '' precipitation static'' phenomena are due to the equalization of charge between the aircraft and its imme- diate surroundings. Aircraft in general become charged in flight, due to such causes as friction between the aircraft skin and atmospheric particles of smoke, i.e., snow, dust, mist or salt, and to a Ie3ser extent due to ionization of exhaust gases, and direct transfer of charge from raindrops and snow. The potential may reach several million volts and corona discharges appear on projecting portions of the air- craft. The total discharge currents will not exceed a few milliamperes, and the total charge on the aircraft never fig. I. (Left) The spinner of an aircraft which was struck by lightning while towing a glider. Fig. 2. (Below) Enlarged view of the pit marks on the spinner.
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